The present invention relates to a plastic-film capacitor, in particular to a metallized, flat, wound capacitor comprising a moistureproof encapsulation and sheet connecting elements fastened to the frontal sides of the capacitor and projecting through the encapsulation, and a process for the production thereof.
Wound plastic-film capacitors in chip configuration are known, for example, from German Offenlegungsschrift No. 3,320,257, which are encapsulated in thermosetting plastic compression molding compounds and which exhibit connecting elements of thin sheet metal at their end faces. The disclosed use of encapsulation and sheet metal connecting elements, respectively, serves to protect the sensitive wound capacitor body against stresses that arise during dip soldering, when the entire wound capacitor body comes into direct contact with liquid tin at a temperature of approximately 260 degrees C., during a soldering time of approximately 5 seconds. The thermosetting encapsulation, which remains dimensionally stable at the temperature of soldering, prevents a bulging of the wound capacitor body, which results from the trapping of air between the individual wound layers and the shrinking of the stretched plastic films. The connecting elements, comprised of thin sheet metal having a thickness of preferably 0.1 mm or more, form a good heat resistor with respect to the wound capacitor body.
In the case of plastic-film capacitors, however, the above-described technique of encapsulating chip-configured capacitors with thermosetting plastic compression molding compounds has a number of shortcomings. The encapsulation process must be carried out at high pressures and temperatures between 150 and 180 degrees C.; as a result, for example, in the case of superthin film capacitors of polyester, there may occur a shrinking of the plastic films and heat damage to the dielectric material. In the case of metallized wound capacitors capable of regeneration, the increased layer pressure in the wound capacitor body produced by the encapsulation pressure also brings about an impairment of the regeneration capability and, consequently, the risk of a reduced insulation resistance.
For small components such as chip capacitors, the efficiency of this encapsulation technique is also impaired by the extremely ineffective usage of the encapsulating material. Encapsulation in the end product incorporates less than 10 percent of the encapsulation material actually used; the remainder of the encapsulating material accumulates as spiderlike sprue. But a repetition of the injection molding of the spiderlike sprue material is out of the question, since a thermosetting material is involved. Consequently, after the processing, more than 90 percent of the expensive plastic compression molding compound accumulates as unavoidable scrap.